Researchers at Japan’s Tohoku University may have found a clue to the irrational response consumers have to free things.

According to Professor Hajime Mushiake, a study of monkeys showed that there are neurons in a part of their brain that react strongly to “zero” in a numerical continuum. This shows that monkeys can recognize the concept of zero, or the absence of quantity, even without a visible symbol for it, the researchers said. It was the first time such neurons were found, they said.

Two macaques were brought in for the study, which was conducted using a screen showing a number of dots. According to Prof. Mushiake, they were trained to solve an easy math calculation while their brain activities were recorded. There were specific neurons that reacted only when the answer was zero and the screen went blank.

“This study was conducted in order to research how the brain works when we add or subtract a number from a number,” Prof. Mushiake told Japan Real Time. The neurons that only responded to zero may be a “precursor of non-verbal concept of zero in primates,” the researchers said in their report.

The study was published earlier this month in Scientific Reports. Prof. Mushiake, an expert on neurophysiology, said the findings could be applied to the field of behavioral economics, and to understand why a person is often attracted to free things from a scientific perspective.

“When people hear that something is free, they sometimes take irrational actions. The neurons may be somehow related to this behavior,” he said.